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5 Tertiary sediments in the basin are approximately 5,000 feet thick, and ,consist of four (4) major stratigraphic units. The units in ascending order are the Ohio Creek Formation (Paleocene, Wasatch Formation (Paleocene-Eocenel, Green River Formation (Eocene), and the Uinta Formation (Eocene). Of these four units, the Green River Formation is the most important in regard to the solution mining of nahcolite. The Green River Formation is divided into four (4) members; the Douglas Creek, Garden Gulch, Anvil Points, and Parachute Creek. The Parachute Creek member is the thickest and most economically important unit in the Green River Formation; it has a large areal extent and contains all of the commercial resources of oil shale, nahcolite, dawsonite, and halite. This member ranges in thickness from less than 200 feet south of the Colorado River to more than 2,000 feet~.n.the north-central part of the ~ basin. The approximate thickness of the Parachute member beneath ,a1`~ ~~ the American Soda leases is.•.1, 495" fee _QiL shale is the ~a;~"~ l~' e+'~ dominate lithology; other rock' types ihclude marlstone, ~ °N~''n9 y nahcolite, dawsonite, and halite. Planned solution minin t~~,,"'S' activity on American Soda's Yankee Gulch Joint Venture odium leases will primarily involve only the upper part o he Green River Formation, specifically the Parachute ee ember. The 7ty Parachute Creek member can be divided into two major units, the .~ ~.. Saline zpne (766 feet thick) and the Leache zone (356 feet r a ~e~~. thick)(k~~y The Leached zone is the result of dissolution of saline ,c~°r; t'+ minerals byLownward percolation of ground water. The main F~~y'~.r '_ stratigraphic section of saline minerals (i.e., the Saline zone) reaches a maximum thickness of about 1,100 feet. The Saline zone contains the nacholite resources of primary interest to American Soda. The Saline zone is a complex array of bedded halite deposits and bedded and non-bedded nahcolite occurrences. At the base of the Leached zone is a layer referred to as the Dissolution Surface which represents the lowermost penetration of ground water into the Parachute Creek member, and marks the contact between the Leached and Saline zones. Hydrogeology: Underground sources of drinking water (USDWs) are defined in 40 CFR § 144.3 as an aquifer or its portion: (a)(1) which supplies any public water system; or (a1 (2) / which contains a sufficient quantity of ground water to supply a public water system; and (i! currently supplies drinking water for human consumption; or (ii) /contains fewer than 10,000 mg/1 total dissolved solids; and